1. Prior Art
U.S. Pat. No. Banczak; 4,021,252; May, 1977 PA0 U.S. Pat. No. Wachtel; 4,024,096; May, 1977 PA0 U.S. Pat. No. Parkinson, 4,045,397; Aug. 1977 PA0 U.S. Pat. No. Hwang; 4,070,322; Jan. 1978 PA0 U.S. Pat. No. Keating; 2,356,065; Aug. 1944 PA0 U.S. Pat. No. Zabiak; 3,687,887; Aug. 1972 PA0 U.S. Pat. No. Ostergren; 3,846,141; Nov. 1974 PA0 U.S. Pat. No. Miyajima; 3,912,520; Oct. 1975 PA0 U.S. Pat. No. Hertz; 3,994,736; Nov. 1976 PA0 U.S. Pat. No. Sambucetti; 4,026,713; May, 1977
Edds et al, "Smear Resistant Jet Ink", IBM Technical Disclosure Bulletin, Vol. 61, No. 6, November 1973.
2. Field of Invention
Metal cans constitute a very widely utilized medium for the protective packaging of a great variety of products, many millions of cans being used daily for packaging of foods, beverages and many other materials. For many years, the common metal can was constructed of tinplated steel, and was customarily referred to in the industry as an ETP can, the initials standing for "electro-tin-plated". Containers of this style are referred to by the public at large as "tin cans". In recent years, however, the metal can industry has developed organic polymeric resinous coatings for metal cans which offer substantially the same protection to the steel as the traditional coating of metallic tin. These organic resin based coatings have been applied both as a top coat over the traditional thin tin coating and as the sole protective coating composition applied directly to the steel can body and/or end components to yield both resin coated ETP cans and resin coated untinned steel cans which are now referred to in the industry as "tin-free steel" or TFS cans.
Manufacturing and processing concerns which package various products in metal cans have found it highly desirable to print, at some point on the can surface, a series of coded symbols which carry information of interest primarily to the packager including the particular machine on which the can was packed, the date and time of packing and perhaps even the identity of the machine operator. Such data are useful in case it is necessary to trace any particular can or cans after they have been packed.
Many of the products packaged in metal cans are subjected to conditions of high temperature, for example 250.degree. F., and high moisture during pasteurization or sterilization procedures carried out before or after the can is filled with product and sealed. In order to be commercially satisfactory, the coded indicia printed on the cans must be capable of withstanding these processing condtions as well as being resistant to rubbing abrasion.
This invention is directed primarily to ink compositions suitable for printing identifying indicia on the resin coated surfaces of TFS, ETP and coated aluminum cans and can components as well as such metals free of organic resins by means of ink jet printing techniques. The ink compositions of this invention are also suitable for printing indicia on most cellulosic surfaces and most polymeric surfaces including plastic films, laminates and pouches.
In a printing apparatus operating on the so-called "ink jet printing" principle, in general terms, a fluid ink is forced, under pressure, through a very small orifice in an orifice block which contains a piezoelectric crystal vibrating at high frequency (50-100,000 vibrations per second) causing the ink passing through the orifice to be broken into minute droplets equal in number to the crystal vibrations. The minute droplets are passed through a charging area where individual droplets receive an electrical charge in response to an electronic signal, the amplitude of the charge being dependent on the amplitude of the signal. The droplets then pass through an electrical field of fixed intensity, causing a varied deflection of the individual droplets dependent on the intensity of the charge associated therewith, after which the deflected drops are allowed to impinge on the base medium which is to receive the decorative or informative printed indicia. Apparatus suitable for carrying out the ink jet printing process is described in detail in U.S. Pat. Nos. 3,465,350 and 3,465,351, issued Sept. 2, 1969. Several components and devices of such apparatus which come into contact with jet ink compositions are often made of polymeric materials. It is in connection wth an apparatus and process as described in the aforementioned patents that the ink of the present invention is designed to function.
In order to operate satisfactorily in an ink jet printing system, an ink must display a consistent breakup length, drop velocity and drop charge under set machine operating conditions. To achieve these ends, the ink must meet strict requirements with regard to viscosity and resistivity, solubility and compatibility of components, stability and anti-skinning properties and must readily redissolve in a suitable solvent for rapid cleanup of the machine components with a minium of effort. The ink must also be stable, that is tolerant to ambient changes especially in temperature and humidity as for example commonly occur during printing in can processing and filling plants. A lack of such stability or tolerance would require frequent readjustment of printer settings to maintain legible indicia. In order to facilitate cleanup of the apparatus after use, the ink components should be readily soluble in a common solvent medium. This will prevent any gradual buildup of ink residues in the system which could result in malfunction. To prevent excessive swelling failures of plastic jet printer parts, solvents used in the ink must also meet solubility and compatibility requirements since the solvents must not excessively solvate the polymers of which the plastic parts are made.
The ink properties set forth above are primarily established by the requirements of the jet printing apparatus. In addition to these requirements, the ink must possess certain other properties which are specifically related to its intended use in the printing of metal cans and, in particular, can bodies intended for the packaging of foodstuffs and beverages.
For example, the ink must properly wet the can surface, whether coated or uncoated, on which the printed indicia are to appear. If the ink is of such compostion that it fails to readily wet the coated metal surface, the ink will bead up on the surface and fail to adhere properly to it. The problem is particularly accentuated by oily or greasy residues left on the can surface from earlier stages of fabrication of the container. On the other hand, if the ink is of such composition as to wet the coated metal surface too readily, the ink drops will flatten out and spread excessively on the surface, diluting the color intensity of the ink and overlapping the image of adjacent dots making the printed image fuzzy and the characters unintelligible. In addition to the requirements of proper wetting of the surface to be printed, the droplets of ink must adhere strongly to the surface after application and drying so that the printed matter is resistant to both physical rubbing or abrasive action and also is resistant to moisture. The ability of the ink to form and retain a desired image on a TFS or coated ETP or aluminum can surface in the presence of moisture and the ability to resist removal by moisture is of great importance in this application because the metal can surfaces are generally damp before, during and after the printing operation. It is particularly difficult to maintain satisfactory adhesion of the ink to metal cans which are subjected to pasteurization, the combination of moisture and high temperature utilized in this process tending to cause the coloring matter to bleed, and to severly reduce the adhesion of the ink to the can body so that it is readily removed by subsequent rubbing or abrasion.
Several attempts have heretofore been made to provide jet ink compositions meeting all of the above requirements and overcoming the difficulties enumerated. An example of such an ink which is utilizable on coated and uncoated substrates, particularly metal cans and can ends, is disclosed and claimed in U.S. application Ser. No. 910,902, filed on May 30, 1978. Another example is U.S. Pat. No. 4,021,252 issued May 3, 1977 to Daniel Philip Banczak and William Eric Tan and commonly assigned herewith, said patent providing excellent inks of the binder type exhibiting the above discussed properties and which are particularly suitable for use on coated and uncoated aluminum cans. Representative inks therein comprise a colorant, a shellac binder, a glycol solvent binder and a water alcohol blend.
Other examples are inks described and claimed in application Ser. No. 634,507 filed Nov. 24, 1975 of R. L. Germonprez, also commonly assigned herewith. The inks of said application are of the binderless type comprising a colorant and a homogeneous blend of water, a lower aliphatic alcohol, an oxygenated aliphatic or cyclic ketone, a surfactant and an aliphatic hydrocarbon. The ink compositions are designed primarily for use on coated TFS or ETP metal cans and comprise a solvent system so selected as to soften and swell the organic polymer substrate sufficiently to allow penetration of the subsurfaces thereof whereby the indicia printed on the substrate become highly resistant to the abrasion and sterilization process. In said compositions, the function of the aliphatic hydrocarbon is to penetrate the thin layer of oily material which serves as a lubricant on the resin coated metal surfaces of can components in the can forming operations.
The present invention represents an improvement over the above and other conventional inks. It has been found that primarily the presence of n-propyl acetate (hereinafter referred to as NPA) as the major solvent, and, secondarily the use of certain resins therewith in jet ink compositions, not only unexpectly provides binder type ink compositions capable of penetrating varied types of organic coatings and lubricants and properly wetting a large number of substrates including coated and uncoated aluminum or steel, but also overcomes other major shortcomings of current jet ink compositions by providing certain commercially required handling, safety, performance and use characteristics.
Certain conventional binderless and binder-type ink compositions capable of printing on both coated and uncoated metal substrates have been found impractical to conventionally manufacture, store, handle and use, due primarily to the characteristics and the amount of the active solvent employed. In contrast with such inks, the ink compositions of this invention are improvements because they employ NPA as the major, active solvent. In addition to being unexpectedly penetrative, composition component-compatable and operative in the required and desired manners heretofore discussed, NPA renders the ink compositions of this invention commercially usable because it does not form explosive peroxides upon exposure to air or oxygen, and has a flash point (60.degree. F., closed cup) which is high enough to allow ink composition manufacture, storage, handling and use at usual ambient temperatures without need of special explosion preventative equipment and measures. Further, NPA is safer to use than such ink compositions since with respect to hazards to workers such as ingestion and inhalation, it is rated in active-systemic Sax Hazard Toxicity Class 2, as compared to Class 3 for other materials which could be employed as the major active solvent. With respect to its evaporative characteristics, NPA's volatility range of 2.7 as compared to n-butyl acetate, allows printed indicia on can components to dry rapidly, within 15 seconds at ambient temperatures, and thereby preclude subsequent smearing due to handling, yet not so rapidly as to present jet printer nozzle plugging or other printer apparatus operating or cleaning difficulties.
Further, it has been found that certain solvents used in jet ink compositions are unfortunately highly solvatable of the polymeric parts such as the tubing often used in jet ink printer apparatus. This causes such parts to excessively and problematically swell or fall. Contrastingly, NPA is less solvatable as to polymers of which such parts are often made and thereby reduces the possibilities of their swelling or failure.
Still further, the ink compositions of this invention have proven significantly more stable than most conventional jet inks which require frequent jet printer adjustments as to operating pressures and crystal drive voltages, due to minor changes in ambient temperature and humidity.